Achnanthidium undetermined

Achnanthidium sp. (Figs 317–327) LM observations:— Valves linear-elliptic to linear-lanceolate, with broadly rounded apices to subrostrate (Figs 317– 327); 7.4–11.6 µm long, 3.5–4.1 µm wide. Raphe valve: axial area narrow and linear, central area small rounded, with 1–2 shortened striae on both sides of the valve (Figs 317–321). Raphe filiform straight. Transapical striae slightly radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 24–28 in 10 µm (Figs 317–321). Rapheless valve: axial area narrow and linear, widening towards the central area, central area absent or lanceolate (Figs 322–327). Transapical striae radiate throughout the entire valve, becoming denser and strongly radiate towards the apices; 23–27 in 10 µm (Figs 322–327). Taxonomical remarks:— Achnanthidium modestiformis (Lange-Bertalot) Van de Vijver (in Van de Vijver et al. 2002: 17) presents similar morphometric features (12–17 µm long, 3.5–4 µm wide, stria density 24–25/10 µm, as Achnanthes modesta Manguin (in Bourrelly & Manguin 1954: 22). However, the Brazilian population has broadly rounded to rostrate apices, whereas they are more capitate in A. modestiformis. Moreover, Achnanthidium sp. has shorter valves. Another similar specimen cited in Brazilian literature is Achnanthes acares M.H. Hohn & Hellerman (1963: 273) (Schneck et al. 2008). The illustrations presented by Hohn & Hellerman (1963) concerning this taxon show a different central area, elliptical in the raphe valve and coarse striae resembling Planothidium. Krammer & Lange-Bertalot (1991) illustrated the holotype of this species using LM, however, they only showed the raphe valves of the specimens, which probably do not correspond to a monoraphid taxon. Additionally, another Brazilian population resembled ours, but it was not identified at the species level (Santos et al. 2011, Figs 21–24, as “ Achnanthidium sp. ”). More studies are necessary concerning the identity of this taxon. Distribution and ecological information:— Achnanthidium sp. was rare (occurrence of
Published as part of Costa, Lívia F., Wetzel, Carlos E., Maquardt, Gisele C., Zanon, Jaques E., Ector, Luc & Bicudo, Denise C., 2022, Taxonomy and ecology of Achnanthidium (Bacillariophyta, Achnanthidiaceae) from southeastern Brazil with the description of six new species, pp. 187-223 in Phytotaxa 575 (3) on pages 214-215, DOI: 10.11646/phytotaxa.575.3.1, http://zenodo.org/record/7431242
{"references":["Van de Vijver, B., Frenot, Y. & Beyens, L. (2002) Freshwater diatoms from Ile de la Possession (Crozet Archipelago, Subantarctica). Bibliotheca Diatomologica 46: 1 - 412.","Bourrelly, P. & Manguin, E. (1954) Contribution a la flore algale d'eau douce des Iles Kerguelen. Memoires de l'Institut Scientifique de Madagascar, series B 5: 5 - 58.","Hohn, M. H. & Hellerman, J. (1963) The taxonomy and structure of diatom populations from three Eastern North American rivers using three sampling methods. Transactions of the American Microscopical Society 82 (3): 250 - 329. https: // doi. org / 10.2307 / 3223932","Schneck, F., Torgan, L. C. & Schwarzbold, A. (2008) Diatomaceas epiliticas em riacho de altitude no sul do Brasil. Rodriguesia 59 (2): 325 - 338. https: // doi. org / 10.1590 / 2175 - 7860200859205","Krammer, K. & Lange-Bertalot, H. (1991) Bacillariophyceae 4. Teil: Achnanthaceae, Kritische Erganzungen zu Navicula (Lineolatae) und Gomphonema. Gesamtliteraturverzeichnis Teil 1 - 4. In: Ettl, H., Gartner, G., Gerloff, J., Heynig, H. & Mollenhauer, D. (Eds.) Susswasserflora von Mitteleuropa Vol. 2 / 4. Gustav Fischer Verlag, Stuttgart-Jena, pp. 1 - 437.","Santos, E. M., Tremarin, P. I. & Ludwig, T. A. V. (2011) Diatomaceas perifiticas em Potamogeton polygonus Cham. & Schltdl.: citacoes pioneiras para o estado do Parana [Periphytic diatoms on Potamogeton polygonus Cham. & Schltdl.: first records from Parana State]. Biota Neotropica 11 (3): 303 - 315. https: // doi. org / 10.1590 / S 1676 - 06032011000300025"]}